This invention relates to a coaxial engine starter and, more particularly, to a coaxial engine starter with a planetary speed reduction gear unit.
Japanese Utility Model Laid-Open No. 63-136262 illustrates one type of the coaxial engine starter in which through bolts for securing a d.c. motor, a solenoid switch and a front machine frame together are positioned to extend outside of the starter housing. Another coaxial engine starter is illustrated in Japanese Utility Model Laid-Open No. 63-168276 in which securing through bolts are positioned inside of the starter housing.
In the coaxial engine starter of the first type in which the through bolts are positioned outside of the housing, the diameter of the inner gear of the planetary speed reduction gear unit disposed within the front machine frame can be made large, so that the number of teeth of the inner gear and accordingly the speed reduction ratio of the planetary speed reduction gear unit can be made large. On the other hand, the through bolts disposed outside of the machine frame impose limitations in the mounting arrangement of the starter on the engine because the outside through bolts decrease the degree of freedom in designing the starter.
In the coaxial engine starter of the second type in which the through bolts are disposed within the machine frame, the diameter of the inner gear of the planetary speed reduction gear unit must be made small by the amount corresponding to the through bolts, so that the speed reduction ratio cannot be made large.
Generally, the relationship between the volume of the armature of a d.c. electric motor and the speed reduction ratio between the d.c. motor and an engine can be expressed by the following equation: EQU Da.sup.2 Lc.varies.Te/g.I..sqroot.Rs.sup.--
where;
Da: outer diameter of an armature core of a d.c. motor PA0 Lc: axial dimension of the armature core PA0 Te: engine torque PA0 g: gear ratio PA0 I: driving current PA0 Rs: resistance of the starter motor
As apparent from the above equation, the volume of the armature is in reverse proportion to the gear ratio or the speed reduction ratio of the planetary speed reduction gear unit, so that decreasing the volume of the armature is extremely difficult when the above speed reduction ratio cannot be increased. Particularly with the coaxial engine starter, since armature rotary shaft forming a sun gear of the planetary speed reduction gear unit is hollow and provided with an output rotary shaft therein, the diameter of the pitch circle of the sun gear must be greater than a predetermined value when taking the mechanical strength of the output rotary shaft and the tooth deddendum of the sun gear, so that the speed reduction ratio cannot be made large when the diameter of the inner gear is limited. Therefore, the resulting engine starter has large overall dimensions and is heavy, posing a problem in the layout for mounting the starter to the engine and in reducing weight.